Add Bolt as a Payment Method Option (Solidus 3.0)

This commit adds bolt as a payment method option during solidus installation.

The install_generator.rb file has been updated to add Bolt as an optional payment method during solidus installation.
The Bolt option in the payment method installs the solidus_bolt gem.

Ref solidusio/solidus_bolt#109

core/lib/generators/solidus/install/install_generator.rb

@@ -10,6 +10,7 @@ class InstallGenerator < Rails::Generators::Base
 
     PAYMENT_METHODS = {
       'paypal' => 'solidus_paypal_commerce_platform',
+      'bolt' => 'solidus_bolt',
       'none' => nil,
     }